1. Field of the Invention
The present invention relates to biaxially oriented polyester films and their production methods. More specifically, it relates to biaxially oriented polyester films that are high in rigidity in all directions within the film plane, high in dimensional stability, and resistant to deformation under load, and serves as a base film for high density magnetic recording media that shows particularly high travelling durability when used as data-recording tape and improved preservability in tape""s service environment, and it also relates to their production methods.
2. Description of the Prior Art
Recent magnetic recording tapes have become thinner and higher in recording density to permit the production of smaller products with longer recording time, and there are increased demands for tape products with smaller elongational deformation under tensile stress and longer preservability in tape""s service environment. Under such conditions surrounding the development of magnetic recording tape products, there are increased demands for improved base film materials that are higher in strength as well as form stability and dimensional stability in the tape""s service environment.
To provide base films that meet the above requirements, aramid materials have been used conventionally as they are high in strength and dimensional stability. Though they are high in price and disadvantageous in terms of cost, manufacturers have had to use them as there are no alternatives. On the other hand, in conventional methods for producing a high-strength biaxially oriented polyester film, film is once stretched in two directions, i.e., machine and transverse directions, and further stretched in the machine direction to ensure high strength in the machine direction (for example, JP-B-SHO 42-9270, JP-B-SHO 43-3040, JP-A-SHO 46-1119 and JP-A-SHO 46-1120). For additional increase in strength in the transverse direction, xe2x80x9clongitudinal and transverse re-stretching processesxe2x80x9d in which film is first re-stretched in the machine direction and then re-stretched in the transverse direction have been proposed (for example, such films are proposed in JP-A-SHO 50-133276 and JP-A-SHO 55-22915). High-strength polyester films produced by these conventional methods have such disadvantages as: 1) tape breaks during use, 2) insufficient rigidity in the transverse direction causes edge damage, 3) elongational deformation under stress or dimensional deformation due to environmental conditions result in a shift of recording tracks that cause errors when records are read out, and 4) insufficient strength brings about difficulty in thickness reduction and makes it impossible to achieve required magnetic conversion performance. Thus, many problems still remain to be solved to allow large-capacity, high-density magnetic recording tape to be produced from these films.
Further, another stretching method has been proposed in which preliminary stretching is performed prior to the above-mentioned stretching-orientation process. U.S. Pat. No. 5,409,657, for example, proposes a process in which film is subjected to preliminary stretching at a draw ratio of 1.2 to 3 times in the machine direction at temperatures of (polyester""s glass transition temperature Tg+40)xc2x0 C. to (crystallization temperature Tcxe2x88x9220)xc2x0 C. followed by stretching in the transverse direction and the machine direction, and shows films that are strengthened only in the longitudinal direction. Further, JP-A-HEI 9-300455 also proposes (a) a process in which film is subjected to preliminary stretching at a draw ratio of 1.5 to 2.5 times in the transverse direction at temperatures of 100xc2x0 C.-120xc2x0 C., followed by stretching in the transverse direction and the machine direction, and (b) a process in which a preliminary stretching is performed at a draw ratio of 1.1 to 2.2 times in the machine direction at temperatures of 100xc2x0 C.-120xc2x0 C. in addition to the above-described process (a), and shows films that are strengthened only in the transverse direction. Furthermore, JP-A-SHO 58-145421 proposes a process in which film is stretched in the two directions simultaneously at a temperature of 115xc2x0 C. or higher, followed by simultaneous biaxial stretching, with the aim of producing thin films and increasing the production speed, and shows films with small Young""s modulus. Films produced by these technologies, however, are not high in rigidity in all directions and cannot solve the problems associated with applying the material to the production of high-density recording tape.
It is an object of the present invention to provide biaxially oriented polyester films that are high in rigidity in all directions in the film plane, high in dimensional stability, and resistant to deformation under load, and serves as base film for high density magnetic recording media that show particularly high travelling durability when used as data-recording tapes and improved preservability in tape""s service environment, and provide their production methods.
The present inventors have carried out studies to solve these problems, and achieved the invention after finding that film having a certain structure and certain physical properties after biaxial stretching and thermal treatment can serve to produce polyester magnetic recording tapes with reduced edge damage, increased travelling durability and improved preservability.
A biaxially oriented polyester film according to the present invention is 7.0 GPa or more in at least either the Young""s modulus in the machine direction (YmMD) or in the transverse direction (YmTD), and in the range of 55xc2x0 or more and 85xc2x0 or less in the circumferential half-width of the diffraction line from the crystal plane in the direction of the polyester""s backbone chain that is determined through crystal orientation analysis by wide angle X-ray diffractometry performed while rotating the polyester film around its normal.
Films as proposed by the present invention have such favorable embodiments as described below:
(a) The crystal size in the polyester""s backbone chain direction is 45 xc3x85 or more and 90 xc3x85 or less.
(b) The sum of the Young""s modulus in the machine direction (YmMD) and that in the transverse direction (YmTD) is 13 GPa or more and 25 GPa or less, and the Young""s modulus in an diagonal direction (45xc2x0 or 135xc2x0) is 6 GPa or more and 10 GPa or less.
(c) The creep compliance after being left for 30 minutes under the conditions of a temperature of 50xc2x0 C. and a load of 28 MPa is 0.11 GPaxe2x88x921 or more and 0.35 GPaxe2x88x921 or less.
(d) The propagating tear strength of the film, converted to 5 xcexcm thickness, in the transverse direction is 0.7 g or more and 1.8 g or less.
(e) The polyester is polyethylene terephthalate.
(f) At least either the ratio R1 (=IMD/IND) of the peak intensity in the machine direction (IMD) to that in the normal direction (IND) at 1615 cmxe2x88x921 measured by laser Raman scattering or the ratio R2 (=ITD/IND) of the peak intensity in the transverse direction (ITD) to that in the normal direction (IND) is 6 or more.
(g) The refractive index in the normal direction (nZD) is 1.470 or more or 1.485 or less, and the planar orientation index (fn) is 0.175 or more and 0.195 or less.
(h) The density of the film is 1.385 or more and 1.400 or less.
(i) The heat shrinkage starting temperature of the film is 70xc2x0 C. or more, and the heat shrinkage at the temperature of 80xc2x0 C. is 0.5% or less.
Such biaxially oriented polyester films according to the present invention as described above serve favorably as base films for high density magnetic recording media, electrostatic capacitors, and thermal transfer ribbons.
Desirable polyester film production methods according to the present invention include, but not limited to, production method (I) and production method (II) described below.
Production method (I) is a biaxially oriented polyester film production method wherein substantially amorphous polyester film is stretched biaxially in the machine direction and the transverse direction so that the birefringence (xcex94n) and the crystallinity of the film become 0-0.02 and 6% or less, respectively, and then the film is subjected to second transverse stretching at a temperature lower than the temperature for the preceding transverse stretching, followed by second longitudinal stretching.
Production method (I) for producing biaxially oriented polyester films of the invention shall have the favorable embodiments as described below:
(a) The ratio (A/B) of the maximum thickness of the edge part of the substantially amorphous polyester film (A) to the thickness at the center of width (B) is in the range of 2.0-6.0.
Production method (II) is a biaxially oriented polyester film production method that comprises three stretching steps, wherein in the first step, non-stretched cast film is stretched biaxially in the machine direction and the transverse direction simultaneously at a temperature in the range of (polyester""s glass transition temperature Tg+25)xc2x0 C. to (Tg+45)xc2x0 C. and an area draw ratio of 2 to 7 times, and subsequently in the second step, the film is stretched biaxially in the machine direction and the transverse direction simultaneously at a temperature in the range of (Tgxe2x88x9215)xc2x0 C. to (Tg+10)xc2x0 C. and an area draw ratio of 4 to 16 times, and in the third step, the film is further stretched biaxially in the machine direction and the transverse direction simultaneously at a temperature in the range of (polyester""s melting point Tmxe2x88x92130)xc2x0 C. to (Tmxe2x88x9210)xc2x0 C. and an area draw ratio of 1.5 to 5 times.
Production method (II) for producing biaxially oriented polyester films of the invention shall have the favorable embodiments as described below:
(a) The stretching in the third step is performed in two or more stages of temperature ranges.
(b) The temperature of the film grips that hold the edge of the film is in the temperature range of (polyester""s glass transition temperature Tg+15)xc2x0 C. to (Tg+50)xc2x0 C.
(c) The film produced by the simultaneous biaxial stretching in the first step has a birefringence (xcex94n) of 0-0.02 and a crystallinity of 6% or less.
The invention is explained hereinafter in more detail together with the preferred embodiments.
The polyesters referred to in the present invention are defined as polymers that are prepared through the condensation polymerization of a diol and a dicarboxylic acid. Dicarboxylic acids include terephthalic acid, isophthalic acid, phthalic acid, naphthalene dicarboxylic acid, adipic acid, and sebacic acid. Diols include ethylene glycol, trimethylene glycol, tetramethylene glycol, and cyclohexane dimethanol. Specifically, useful polyesters include polymethylene terephthalate, polyethylene terephthalate, polypropylene terephthalate, polyethylene isophthalate, polytetramethylene terephthalate, polyethylene-p-oxybenzoate, poly-1,4-cyclohexylene dimethylene terephthalate, and polyethylene-2,6-naphthalate. Needless to say, these polyesters may be either homopolymers or copolymers, with the copolymerization components being, for example, such diol components as diethylene glycol, neopentyl glycol, and polyalkylene glycol, and such dicarboxylic acid components as adipic acid, sebacic acid, phthalic acid, isophthalic acid, and 2,6-naphthalene dicarboxylic acid. From the viewpoint of mechanical strength, thermostability, chemical resistance, and durability, favorable ones for the invention include polyethylene terephthalate, polypropylene terephthalate, polyethylene isophthalate, polyethylene naphthalate (polyethylene-2,6-naphthalate), and their copolymers. In particular, polyethylene terephthalate is highly desirable for the invention in terms of film characteristics and price. The intrinsic viscosity (IV) of the polyester to be used should preferably be in the range of 0.6 dl/g or more and 1.0 dl/g or less, with the range of 0.65 dl/g or more and 0.80 dl/g or less being particularly desirable from the viewpoint of film forming ability, dimensional stability, and tear resistance.
Any biaxially oriented polyester film of the invention shall be 7.0 GPa or more at least in either the Young""s modulus in the machine direction (YmMD) or that in the transverse direction (YmTD), and shall be in the range of 55xc2x0 or more and 85xc2x0 or less in the circumferential half-width of the diffraction line from the crystal plane in the direction of the polyester""s backbone chain that is determined through crystal orientation analysis by wide angle X-ray diffractometry performed while rotating the polyester film around its normal. The circumferential half-width of the diffraction line from the crystal plane in the direction of the polyester""s backbone chain represents the broadening of the distribution of the orientation directions of the crystals in the biaxially oriented polyester film. If the half-width is less than 55xc2x0, the propagating tear strength of the film will be small, easily causing tape breakage, whereas if it is more than 85xc2x0, it will be impossible to produce a film that is strong in all directions in the film plane, and the goal of the invention will not be achieved. The crystal plane in the direction of the polyester""s backbone chain as referred to herein is the crystal plane that is detected as a diffraction line by wide angle X-ray diffractometry with its normal being nearer to the polyester""s backbone chain than any other crystal plane. It is (xe2x88x92105) plane for polyethylene terephthalate and (xe2x88x92306) plane for polyethylene-2,6-naphthalate. The above-mentioned half-width should preferably be in the range of 60xc2x0 or more and 85xc2x0 or less, most desirably in the range of 65xc2x0 or more and 80xc2x0 or less, to achieve the effect of the invention.
If both the Young""s modulus in the machine direction (YmMD) and that in the transverse direction (YmTD) of the Young""s modulus is less than 7.0 GPa, the film will be too small in rigidity, and thin film produced from it will easily suffer elongational deformation under stress (especially in the machine direction) and edge damage (especially in the transverse direction). Either of the Young""s modulus values, YmMD and YmTD, should more preferably be 8 GPa or more from the viewpoint of the tape""s elongational deformation and edge damage.
For the present invention, the crystal size in the film in the polyester""s backbone chain direction should preferably be in the range of 45 xc3x85 or more and 90 xc3x85 or less. Here, the polyester""s backbone chain direction is defined as the direction of the normal of a crystal plane that is nearest to the direction of the polyester""s backbone chain. It is the direction of the normal to the (xe2x88x92105) plane for polyethylene terephthalate and (xe2x88x92306) plane for polyethylene-2,6-naphthalate. If the crystal size is less than 45 xc3x85, the resulting tape will suffer a large elongational deformation, edge damage, and stability during storage of manufactured tape. If the crystal size is more than 90 xc3x85, tape breakage will take place at an increased frequency. The desirable crystal size depends on the polyester used. For polyethylene terephthalate, it should preferably be in the range of 50 xc3x85 or more and 85 xc3x85 or less, more preferably in the range of 55 xc3x85 or more and 80 xc3x85 or less. If the polyester used is polyethylene-2,6-naphthalate, the range of 50 xc3x85 or more and 65 xc3x85 or less is further more desirable.
For the films of the present invention, the sum of the Young""s modulus in the machine direction (YmMD) and that in the transverse direction (YmTD), i.e., YmMD+YmTD, should preferably be in the range of 13 GPa or more and 25 GPa or less, and the Young""s modulus in the diagonal direction should preferably be in the range of 6 GPa or more and 10 GPa or less. The Young""s modulus in the diagonal direction referred to above is defined as the Young""s modulus in the film plane in the direction of 45xc2x0 or 135xc2x0 assuming that the machine direction and the transverse direction of the film are in the direction of 90xc2x0 and 0xc2x0, respectively. If said sum of the Young""s modulus values is less than 13 GPa and the Young""s modulus in the diagonal direction is less than 6 GPa, the film is likely to suffer elongation deformation under stress. Conversely, if said sum of the Young""s modulus values is more than 25 GPa and the Young""s modulus in the diagonal direction is more than 10 GPa, the film is likely to suffer deterioration in tear resistance and heat shrinkage properties, making it difficult to achieve the effect of the invention. The sum of the Young""s modulus in the machine direction (YmMD) and that in the transverse direction (YmTD), i.e., YmMD+YmTD, should more preferably be in the range of 14 GPa or more and 20 GPa or less, and the Young""s modulus in the diagonal direction should more preferably be in the range of 7 GPa or more and 9 GPa or less. Though depending on the rigidity of the magnetic coat over the base film and the service conditions of the tape, the ratio of YmMD and YmTD, i.e., YmMD/YmTD, should preferably be in the range of 0.6-1.3, more preferably in the range of 0.7-1.2, in order to reduce edge damage. If a magnetic layer is added to the base film to increase the rigidity, YmMD should preferably be 6.0 GPa or more, and YmMD/YmTD should preferably be in the ranges of 0.6-0.9.
For the present invention, the creep compliance of the film left for 30 minutes under the condition s of a temperature of 50xc2x0 C. and a load of 28 MPa should preferably in the range of 0.11 GPaxe2x88x921 or more and 0.35 GPaxe2x88x921 or less. For the invention, if the creep compliance is more than 0.35 GPaxe2x88x921, the tension that takes place during travelling or storage of the tape will likely to cause elongational deformation of the tape, leading to shifts of tracks during data recording. Conversely, if the creep compliance is less than 0.11 GPaxe2x88x921, tape breakage will take place frequently. For the present invention, the creep compliance should more preferably in the range of 0.15 GPaxe2x88x921 or more and 0.30 GPaxe2x88x921 or less. The creep compliance for this invention is as defined on p.150 in xe2x80x9cKobunshi-kagaku Joron (An Introduction to Polymer Chemistry) 2nd Ed.xe2x80x9d published by Kagakudojin.
For the biaxially oriented polyester films of the invention, the propagating tear strength of the film, converted to 5 xcexcm thickness, in the transverse direction should preferably in the range of 0.7 g or more and 1.8 g or less. For the invention, furthermore, the propagating tear strength in the transverse direction should more preferably in the range of 0.8 g or more and 1.5 g or less.
The polyester to be used for the invention should preferably be polyethylene terephthalate, as described above, and for the film of the invention in this case, at least either the ratio R1 (=IMD/IND) of the peak intensity in the machine direction (IMD) and that in the normal direction (IND) at 1615 cmxe2x88x921 measured by laser Raman scattering or the ratio R2 (=ITD/IND) of the peak intensity in the transverse direction (ITD) and that in the normal direction (IND) should preferably be 6 or more. The ratio R1 (=IMD/IND) of the peak intensity in the machine direction (IMD) and that in the normal direction (IND) at 1615 cmxe2x88x921 measured by laser Raman scattering and the ratio R2 (=ITD/IND) of the peak intensity in the transverse direction (ITD) and that in the normal direction (IND) are related to the intensity of orientation in the machine direction and the transverse direction, respectively. However, the 1615 cmxe2x88x921 Raman band used for the invention is attributed to the Cxe2x95x90C stretching vibration (xcexdCxe2x95x90C) of the benzene ring, and its intensity depends on the packing of benzene rings. For biaxially oriented film, in particular, the intensity depends significantly on factors other than the orientation. For the present invention, in order to produce film highly strengthened in all directions in the film plane, at least either of the intensity ratios for the machine direction and the transverse direction, R1 and R2, should preferably be 6 or more, more preferably 7 or more. For the production of magnetic recording media, in particular, it is further more preferable that the intensity ratio for the transverse direction, R2, is 6 or more.
For biaxially oriented polyethylene terephthalate films of this invention, the refractive index in the normal direction (nZD) should preferably be in the range of 1.470 or more and 1.485 or less, and the planar orientation index (fn) should preferably in the range of 0.175 or more or 0.195 or less. For the invention, if the refractive index in the normal direction (nZD) is more than 1.485 and the planar orientation index (fn) is less than 0.175, elongational deformation will become likely to be caused during the travelling of magnetic tape by the stress applied on the tape, leading to shifts of tracks. If the refractive index in the normal direction (nZD) is less than 1.470 and the planar orientation index (fn) is more than 0.195, the propagating tear strength of the film will be small, and tape breakage will become likely to occur. For the film of this invention, the refractive index in the normal direction (nZD) should more preferably be in the range of 1.473 or more and 1.482 or less, and the planar orientation index (fn) should more preferably in the range of 0.180 or more or 0.193 or less.
For biaxially oriented polyethylene terephthalate films of this invention, the density should preferably be in the range of 1.385 or more and 1.400 or less. For the film of this invention, if the density is less than 1.385, the structures in the film will not be fixed sufficiently, causing deterioration in the preservability of the tape, whereas if density is more than 1.400, the propagating tear strength of the film will be small, leading to frequent occurrence of tape breakage.
For biaxially oriented polyethylene terephthalate films of this invention, the heat shrinkage starting temperature should preferably be 70xc2x0 C. or more and the heat shrinkage at the temperature of 80xc2x0 C. should preferably be 0.5% or less from the viewpoint of the elongational deformation and preservability of the tape. It is more preferably that the heat shrinkage starting temperature is 75xc2x0 C. or more and the heat shrinkage at the temperature of 80xc2x0 C. is 0.3% or less. For the present invention, if the heat shrinkage-starting temperature is less than 70xc2x0 C. or if the heat shrinkage at the temperature of 80xc2x0 C. is more than 0.5%, the dimensional stability will be likely to deteriorate, leading to thermal deformation being caused when the magnetic tape is heated by the heat of friction between the travelling magnetic tape and the recording head, or giving rise to deterioration in the preservability of the tape.
Biaxially oriented polyester films of this invention can serve favorably for the production of magnetic recording media, electrostatic capacitors, and thermal transfer ribbons, and their film thickness should preferably be in the range of 0.5-20 xcexcm, depending on their use and purpose. For the production of magnetic recording media, they provide base film suited to high-density magnetic recording tapes, especially to hose for data storage. The magnetic recording density should prefer ably be 30 GB (gigabytes) or more, more preferably 70 GB or more, further more preferably 100 GB or more. The film thickness should preferably be in the range of 1 xcexcm or more and 15 xcexcm or less for the production of conventional magnetic recording media, in the range of 2 xcexcm or more and 10 xcexcm or less for the production of coat-type magnetic recording media for data storage, and in the range of 3 xcexcm or more and 9 xcexcm or less for the production of evaporation-type magnetic recording media for data storage.
For the production of electrostatic capacitors, biaxially oriented polyester films of the present invention should preferably be 0.5-15 xcexcm in thickness, which leads to high stability of dielectric breakdown voltage and dielectric properties.
For the production of thermal transfer ribbons, biaxially oriented polyester films of the present invention should preferably be 1-6 xcexcm in thickness, which permits highly fine printing without suffering wrinkles or causing uneven printing, excessive ink transfer, etc.
For the production of base paper for thermosensitive stencil printing, biaxially oriented polyester films of this invention should preferably be 0.5-5 xcexcm in thickness, which permits easy perforation at low energy, variation of perforation diameters depending on the energy level, and high-quality color printing using several plates.
For biaxially oriented polyethylene terephthalate films of this invention, the surface roughness (Ra) of the magnetic recording surface should preferably be in the range of 0.2-15 nm to reduce the distance between the magnetic head and the magnetic tape to enhance the electromagnetic conversion properties. The surface roughness (Ra) of the travelling surface, which is opposite to the magnetic recording surface, should preferably be in the range of 5-30 nm from the viewpoint of the handling of base film and the winding of film to produce a roll. Controlling the roughness of the two surfaces separately is highly desirable to ensure both high travelling performance and high electromagnetic conversion properties of the tape. This can be achieved by co-extruding two resin materials consisting of polyester and particles with different diameters to produce a laminate film consisting of two or more layers. Thin layer may be further added to the magnetic recording surface to produce a three-layered film. For a two-layered film, the ratio (A/B) of the thickness of the layer that works as the magnetic recording surface (A) and the thickness of the layer that works as the travelling surface (B) should preferably be in the range from 80/1 to 3/1.
Polyester films of the present invention may contain inorganic or organic particles, and other various additives including oxidation inhibitor, antistatic agent, and crystal nucleation agent. They may contain a small amount of other resins such as a copolyester resin whose backbone chain contains mesogenic groups (liquid crystal-forming structure unit).
Such copolyester resins whose backbone chain contains mesogenic groups include copolyester produced from a monooxy-monocarboxylic acid compound, aromatic dihydroxy compound, aromatic dicarboxylic acid, alkylene diol, etc. Such monooxy-monocarboxylic acid compounds include p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid. Such aromatic dihydroxy compounds include 4,4xe2x80x2-dihydroxy biphenyl, hydroquinone, and 2,6-dihydroxy naphthalene. Such aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, 4,4xe2x80x2-diphenyl dicarboxylic acid, 2,6-naphthalene dicarboxylic acid, and 1,2-bis(phenoxyl)ethane-4,4xe2x80x2-dicarboxylic acid. Such alkylene diols include ethylene glycol, and butanediol. The mole ratio (M/N) of the sum (M) of the quantities of the copolymerized monooxy-monocarboxylic ac id compound and the copolymerized aromatic dihydroxy compound and the quantity (N) of the copolymerized alkylene diol should preferably be in the range from 80/20-50/50. Desirable copolyester to be added to polyethylene terephthalate film or polyethylene naphthalate film include those produced from hydroxybenzoic acid, 4,4xe2x80x2-dihydroxy biphenyl, ethylene glycol, terephthalic acid or 2,6-naphthalene dicarboxylic acid, etc. Their contents in the polyester film should preferably be in the range of 0.5-10.0 wt %.
The above-described inorganic particles include, but not limited to, such oxides as silicon oxide, aluminum oxide, magnesium oxide, and titanium oxide, such complex oxides as kaolin, talc, and montmorillonite, such carbonates as calcium carbonate, and barium carbonate, such sulfates as calcium sulfate, and barium sulfate, such titanates as barium titanate, and potassium titanate, and such phosphates as tribasic calcium phosphate, dibasic calcium phosphate, and monobasic calcium phosphate. Two or more of these may be used together to achieve a specific objective
The above-described organic particles include, but not limited to, particles of such vinyl materials as polystyrene, crosslinked polystyrene, crosslinked styrene-acrylic polymers, crosslinked acrylic polymers, crosslinked styrene-methacrylic polymers, and crosslinked methacrylic polymers, as well as such other materials as benzoguanamine formaldehyde, silicone, and polytetrafluoroethylene. Any other particles may be used if at least a part of the particles are fine polymer particles that are insoluble to the polyester. Said organic particles should preferably be spherical and have a uniform diameter distribution in order to ensure high slipperiness and uniform protrusions formed over the film surface.
The desired diameter, content, and shape of the particles depend on the use and objective. Generally, however, their average diameter should preferably be in the range of 0.01 xcexcm or more and 2 xcexcm or less, and their content should preferably be in the range of 0.002 wt % or more and 2 wt % or less.
The above description shows that biaxially oriented polyester films with a specific structure and properties can serve excellently as base film for high density magnetic recording tape because they can reduce tape breakage and enhance the travelling durability and preservability. Desirable methods to produce these biaxially oriented polyester films of this invention are described below. Needless to say, the description given below does not place any limitations on the present invention unless it extends beyond the scope of the invention.
The machine direction, abbreviated as MD, may also be called the longitudinal direction in relation to the stretching process for film formation, while the transverse direction, abbreviated as TD, may also be called the transverse direction in relation to the stretching process for film formation.
The biaxially oriented polyester films of the present invention consist of a melt-molded polyester resin sheet that is oriented by sequential biaxial stretching and/or simultaneous biaxial stretching in the machine direction and transverse direction. Such a sheet is produced by carrying out biaxial stretching several times at different temperatures to achieve a very high orientation. Desirable production methods include, but not limited to, production methods (I) and (II) described above.
For production method (I), sequential biaxial stretching of polyethylene terephthalate (hereafter, referred to as PET) film is described below as an example.
Pellets of polyethylene terephthalate (inherent viscosity: 0.65 dl/g, glass transition temperature Tg: 75xc2x0 C., melting point: 255xc2x0 C.) is dried adequately in a vacuum, fed to an extruder maintained at a temperature of 270-300xc2x0 C., and extruded through a T-die to produce a sheet. This molten sheet is allowed to adhere over a drum with a cooled surface of 10-40xc2x0 C. using static electricity to ensure close contact, so that substantially amorphous non-stretched cast film is obtained. During this procedure, the refractive index in the machine direction and that in the transverse direction should preferably be controlled in the range of 1.570-1.575 while the crystallinity should preferably be maintained at 1.5% or less, more preferably 1.0% or less, in order to produce a film of the present invention. Furthermore the ratio (A/B) of the maximum thickness of the edge part of the non-stretched film (A) and the thickness at the center of width (B) should preferably be in the range of 2.0-6.0, more preferably 3.0-5.0 so that subsequent stretching will be performed favorably.
Required conditions (stretching temperature, degree of stretching) for the sequential biaxial stretching of this non-stretched film must be identified so that the film will become 0-0.02 in birefringence (xcex94n) and 6% or less in crystallinity after the sequential biaxial stretching in the machine direction and transverse direction. For the sequential biaxial stretching to be performed favorably, the non-stretched film should be introduced to a group of heated metallic rolls to achieve 1.5-2.5 times stretching in the machine direction at a temperature in the range of (polyester""s glass transition temperature Tg+15)xc2x0 C. to (Tg+45)xc2x0 C., more preferably (polyester""s glass transition temperature Tg+25)xc2x0 C. to (Tg+45)xc2x0 C. (MD stretching 1). This stretching should preferably be carried out in two steps to achieve the required degree of stretching. The film is held by tenter clips at its ends, brought to a tenter, pre-heated, and stretched at a draw ratio of 1.5-2.5 times in the transverse direction at a temperature in the range of (polyester""s glass transition temperature Tg+15)xc2x0 C. to (Tg+45)xc2x0 C., more preferably (polyester""s glass transition temperature Tg+25)xc2x0 C. to (Tg+45)xc2x0 C. (TD stretching 1) to produce a film that should preferably have a birefringence of 0-0.02, more preferably 0-0.01, further more preferably 0-0.005, and a crystallinity of 6% or less, more preferably 3% or less, further more preferably 2% or less, as determined by a density-based technique. The refractive index of this film in the machine direction and transverse direction should preferably be 1,590 or less, more preferably 1,580 or less. Thus, it is preferred that stretching to 1.5-2.5 times in the machine direction and the transverse direction is carried out at a temperature where orientation and crystallinity are not increased significantly by this stretching. Such stretching disentangles the polymer chains to permit the formation of a structure consisting of oriented benzene rings stacked in twos or threes in the vertical direction (stacking structure). The formation of this structure followed by re-stretching in several steps is desirable to produce a film disclosed herein. The degree of stretching in the machine direction referred to here is defined as the ratio of the film speed after the stretching to that before the stretching in the stretching process, whereas several lines extended in the machine direction and aligned with each other at equal intervals in the transverse direction are provided on the film before the stretching and the increased distance between the lines are measured after the stretching to determine the degree of stretching in the transverse direction that is defined as the ratio of the distance between the lines after the stretching to that before the stretching.
Following TD stretching 1, the film is stretched again in the transverse direction at a temperature below the stretching temperature of TD stretching 1, followed by further stretching in the machine direction. The stretching in the transverse direction should preferably be carried out up to 3-5 times in the transverse direction at a temperature in the range of (Tgxe2x88x9215)xc2x0 C. to (Tg+25)xc2x0 C., more preferably (Tgxe2x88x9215)xc2x0 C. to (Tg+10)xc2x0 C. (TD stretching 2). Since such stretching at a temperature below (Tg+10)xc2x0 C. causes a small degree of necking (pseudo-necking stretching), the draw ratio of stretching should preferably be set to 3 times or more. If the draw ratio of TD stretching 2 is less than 3 times, the film is likely to become less uniform in thickness, which should be avoided carefully. To ensure high-degree stretching at a temperature near Tg, it is important that said MD stretching 1 and TD stretching 1 be carried out under said desired temperature and stretching degree conditions to provide a biaxially stretched film having such desired properties as described above.
For the subsequent second longitudinal stretching, the film is introduced to a group of heated metallic rolls (with hard, chrome plated, mirror finished surface) and re-stretched in the machine direction at a draw ratio of 2-6 times preferably at a temperature in the range of (Tgxe2x88x9225)xc2x0 C. to (Tg+85)xc2x0 C. (MD stretching 2). More preferably, second longitudinal stretching at a draw ratio of 3-6 times (MD stretching 2) should be performed in several steps, specifically, at a temperature in the range of (Tgxe2x88x9215)xc2x0 C. to (Tg+10)xc2x0 C. in the first step and at a temperature in the range of more than (Tg+10)xc2x0 C. up to (polyester""s melting point Tm+85)xc2x0 C. in the subsequent steps. When such multiple-step longitudinal stretching is performed, the degree of stretching in the first step that is carried out at a temperature in the range of (Tgxe2x88x9215)xc2x0 C. to (Tg+10)xc2x0 C. should be in the range of about 70% to 95% of the total draw ratio in the MD stretching 2 process. It is more preferred that this draw ratio of stretching achieved by said machine-direction re-stretching in the fist step that is performed at a temperature of (Tgxe2x88x9215)xc2x0 C. to (Tg+10)xc2x0 C. be achieved in two or more divided steps.
Subsequently, this biaxially oriented film may be further re-stretched in the transverse direction. For this second stretching, the biaxially stretched film is held by tenter clips at its ends, brought to a tenter, pre-heated, and stretched at a draw ratio of 1.05-3 times in the transverse direction at a temperature in the range from the temperature of MD stretching 2 to (Tmxe2x88x9220)xc2x0 C. preferably stretched in one step or multiple steps in the transverse direction at a draw ratio of 1.2-2.5 times with the temperature gradually increased in the range from more than (Tg+10)xc2x0 C. to (polyester""s melting point Tmxe2x88x9245)xc2x0 C. (TD stretching 3). The temperature for TD stretching 3 should preferably be in the range of (Tmxe2x88x92120)xc2x0 C. to (Tmxe2x88x9245)xc2x0 C. where the crystallinity of the film starts to increase. This film is then heat-treated in the temperature range of (Tmxe2x88x9275)xc2x0 C. to (Tmxe2x88x9235)xc2x0 C., and subsequently relaxed in the transverse direction and/or machine direction during the cooling from the heat treatment temperature. The relaxation treatment should preferably be performed in two or more steps (for example at 180-130xc2x0 C. and 130-90xc2x0 C.). Relaxation in the transverse direction may be achieved favorably by gradually decreasing the distance between the guide rails for the tenter clips while relaxation in the machine direction may be achieved favorably by gradually decreasing the distance between the clips that hold the edges of the film.
This film may be subjected to further heat treatment in the temperature range of (Tgxe2x88x9230)xc2x0 C. to (Tg+110)xc2x0 C. Favorable methods for this second heat treatment include the use of a heating oven, and the use of several heating rolls. In a favorable heat treatment process using a heating oven, for example, a tensile stress of 2 MPa or more is applied to a film with edges (thicker parts formed at the ends of the film during its production process) to pull the edge in the machine direction, and the film is stretched in the transverse direction by the expanding apparatus (expanding roll, etc.) provided at the entrance of the heating oven, followed by heat treatment on the nip rollers provided at both sides of the heating oven. In this step, the running speed of the nip roll at the rear side may be set lower than that of the nip roll at the front side to achieve relaxation in the machine direction. When several heating rolls are used for the second heat treatment, nip rolls may be provided at both sides of the group of heating rolls, and the heat treatment is carried out via the nip rolls. In this step, the running speed of the nip roll at the rear side may be set lower than that of the nip roll at the front side to achieve relaxation in the machine direction.
After trimming the edges, the film produced may be slit into tapes which may be wound into a roll and subjected to aging treatment at a temperature of (Tgxe2x88x9230)xc2x0 C. to (Tg+30)xc2x0 C. for 1-10 days.
Examples of production method (II) using simultaneous biaxial stretching are described below. When simultaneous biaxial stretching is applied, the basic film production principles and stretching conditions are the same as those for the sequential biaxial stretching described above, and the sequential biaxial stretching process may be replaced partially or entirely with simultaneous biaxial stretching. For example, 1) MD stretching 1 and TD stretching 1 are carried out by simultaneous biaxial stretching, with the rest of the process being performed by sequential stretching; 2) MD stretching 1, TD stretching 1, and the first step of TD stretching 2 and MD stretching 2 are carried out by simultaneous biaxial stretching, with the rest of the process being performed by sequential stretching; 3) MD stretching 1 and TD stretching 1 are carried out by sequential biaxial stretching, followed by the first step of TD stretching 2 and MD stretching 2, the second step of MD stretching 2, and TD stretching 3 performed by simultaneous biaxial stretching; 4) MD stretching 1, TD stretching 1, TD stretching 2, and the first step of MD stretching 2 are carried out by sequential biaxial stretching, followed by the second step of MD stretching 2, and TD stretching 3 performed by simultaneous biaxial stretching; 5) MD stretching 1, TD stretching 1, the first step of TD stretching 2 and MD stretching 2, the second and subsequent steps of MD stretching 2, and TD stretching 3, i.e., the entire stretching process, are carried out by simultaneous biaxial stretching; 6) MD stretching 1 and TD stretching 1 are carried out by sequential biaxial stretching, and subsequently the first step of TD stretching 2 and MD stretching 2 are carried out by simultaneous biaxial stretching, followed by the second and subsequent steps of MD stretching 2, and TD stretching 3 being carried out by sequential biaxial stretching; or 7) MD stretching 1 and TD stretching 1 are carried out by simultaneous biaxial stretching, and subsequently TD stretching 2 and MD stretching 2 are carried out by sequential biaxial stretching, followed by the rest of the process being carried out by simultaneous biaxial stretching.
For the present invention, the fifth procedure, where MD stretching 1, TD stretching 1, the first step of TD stretching 2 and MD stretching 2, the second and subsequent steps of MD stretching 2, and TD stretching 3, i.e., the entire stretching process, are carried out by simultaneous biaxial stretching, is preferred.
A process where the entire stretching process is carried out by simultaneous biaxial stretching is described below.
Substantially amorphous non-stretched cast film is produced according to the same procedure as described for film production by sequential biaxial stretching. During this procedure, the refractive index in the machine direction and that in the transverse direction should preferably be controlled in the range of 1.570-1.575 while the crystallinity should preferably be maintained at 1.5% or less, more preferably 1.0% or less, in order to produce a film of the present invention. Furthermore, the ratio (A/B) of the maximum thickness of the edge part of the non-stretched film (A) and the thickness at the center of width (B) should preferably be in the range of 2.0-6.0, more preferably 3.0-5.0, further more preferably 3.0-4.0, so that subsequent stretching will be performed favorably.
The cast film, with its edges held by tenter clips, is placed in a simultaneous biaxial stretching machine in which it is stretched at a temperature of (Tg+25)xc2x0 C. to (Tg+45)xc2x0 C. and an area draw ratio of 2-7 times, preferably up to 1.5-2.5 times in both the machine direction and transverse directions. The refractive index in the machine direction and the transverse direction of this simultaneous biaxially stretched film should preferably be 1.590 or less, more preferably 1.580 or less. Its birefringence should preferably be 0-0.02, more preferably 0-0.01, further more preferably 0-0.005. The crystallinity as determined from density measurement should preferably be 6% or less, more preferably 3% or less, further more preferably 2% or less, to provide film of the present invention. Subsequently, the biaxially stretched film is further subjected to simultaneous biaxial stretching at a temperature of (Tgxe2x88x9215)xc2x0 C. to (Tg+10)xc2x0 C. and an area draw ratio of 4-16 times, preferably up to 3-5 times in each of the machine direction and transverse directions. Subsequently, this film is further subjected to one-step or multiple-step simultaneous biaxial stretching at a temperature of more than (Tg+10)xc2x0 C. to (polyester""s melting point Tmxe2x88x9245)xc2x0 C. and an area draw ratio of 1.5-5 times, preferably at a draw ratio of 1.2-2.5 times in each of the machine direction and transverse directions. The temperature for the simultaneous biaxial stretching in this process should preferably be in the range of (Tmxe2x88x92120)xc2x0 C. to (Tmxe2x88x9245)xc2x0 C. where the crystallinity of the film starts to increase. This film is then heat-treated in the temperature range of (Tmxe2x88x9275)xc2x0 C. to (Tmxe2x88x9210)xc2x0 C., preferably (Tmxe2x88x9270)xc2x0 C. to (Tmxe2x88x9235)xc2x0 C., and subsequently relaxed in the transverse direction and machine direction during the cooling from the heat treatment temperature. The relaxation treatment should preferably be performed in two or more steps (for example at 180-130xc2x0 C. and 130-90xc2x0 C.). Relaxation in the transverse direction may be achieved favorably by gradually decreasing the distance between the guide rails for the tenter clips while relaxation in the machine direction may be achieved favorably by gradually decreasing the distance between the tenter clips that hold the edges of the film.
A preferred machine for the simultaneous biaxial stretching for the present invention is the simultaneous biaxial stretching tenter that uses a linear motor to move the tenter clips in the machine direction. The shape of the clip""s face that comes in contact with the film should preferably be such that the ratio (LMD/LTD) of the length in the machine direction (LMD) to the width in the transverse direction (LTD) is in the range of 3-15, which is preferred to achieve uniform stretching in the machine direction at the end of the film. The temperature of the clips that hold the edges of the film for stretching should preferably be in the range of (Tg+15)xc2x0 C. to (Tg+50)xc2x0 C. to ensure uniform stretching in the machine direction at the end of the film. The clips pass through the tenter while being heated in the pre-heating zone, stretching zone, and heat treatment zone, come out of the tenter, and move around the oven to return to the entrance of the tenter, and the temperature of the clips referred to above is defined as that temperature measured prior to holding the edges of the film. The temperature of the clips is controlled by adjusting the cooling air flow rate and the length of the cooling portion in the path for return to the entrance. This temperature becomes stationary after 3-5 hours of continuous operation under the prescribed film production conditions.
For the present invention, the polyester film may be coated with a coating material prior to or after the stretching of the film to provide special surface properties for easy adhesion, high slipperiness, easy mould release, and high antielectricity.
The biaxially stretched polyester films of this invention serve favorably as material for magnetic recording media as well as electrostatic capacitors, thermal transfer ribbons, and heat-sensitive base paper for stencil printing.
(1) Circumferential Half-width of the Diffraction Line from Film""s Crystal Plane Determined by Wide Angle X-ray Diffractometry
This was determined by diffractometry under the following conditions using an X-ray diffractometer.
A stack of specimens of a size of 2 cmxc3x972 cm aligned in the same direction and the counter were placed at the diffraction line from the crystal plane determined from 2xcex8/xcex8 scanning, and the specimen stack was rotated within the plane to provide the circumferential profile (xcex2scanning). The peak""s half-width (deg) was determined from the peak profile obtained from the xcex2 scanning assuming that the bottoms at both sides of the peak constitute the background.
(2) Crystal Size Determined from Wide Angle X-ray Diffractometry
This was determined by the transmission method under the following conditions using an X-ray diffractometer.
A stack of specimens of a size of 2 cmxc3x972 cm aligned in the same direction was treated in collodion ethanol solution into a lump, and subjected to wide angle X-rays diffraction observation to provide 2xcex8/xcex8 intensity data, and the crystal size was calculated from the half-width of planes in different directions using the Scherrer""s equation given below. The crystal size represents the size in the principal orientation axis.
Crystal size L (xc3x85)=Kxcex/xcex20 cos xcex8B
(3) Young""s Modulus
Measurement was performed according to the procedure specified in ASTM-D882 using an Instron type tensile tester. The conditions for measurement are given below.
(4) Creep Compliance
A specimen of a width of 4 mm was cut off, and set in TMA xe2x80x9cTM-3000xe2x80x9d produced by Shinku Riko Corporaltion with a heating control unit xe2x80x9cTA-1500xe2x80x9d so that the effective specimen length was 15 mm.
A load of 28 MPa was applied to the specimen, which was left for 30 minutes under the conditions of 50xc2x0 C. and 65% RH, and the elongation of the film was measured. The film""s elongation (percentage, represented as (xcex94L) was determined with an NEC PC-9801 personal computer via a Canopus Co., Ltd. AD converter ADX-98E, and the creep compliance was determined from the following equation.
Creep compliance (GPaxe2x88x921)=(xcex94L/100)/0.028
(5) Orientation of Film Determined from Laser Raman Scattering
The measuring conditions used for the laser Raman spectrometry are described below.
The film used for the measurement was embedded in polymethyl methacrylate, and wet-polished. The direction of the cross section was set to be parallel to the transverse direction. The central part was used for measurement. Ten measurements were made at slightly different positions, and their average was calculated. The 1615 cmxe2x88x921 band intensity for polarization parallel in the machine direction (IMD) and the 1615 cmxe2x88x921 band intensity for polarization parallel in the normal direction (IND) were measured to determine the ratio R1 (R1=IMD/IND), which represents the degree of orientation. In addition, the 1615 cmxe2x88x921 band intensity for polarization parallel in the transverse direction (ITD) and the 1615 cmxe2x88x921 band intensity for polarization parallel in the normal direction (IND) were measured to determine the ratio R2 (R2=ITD/IND), which represents the degree of orientation.
(6) Propagating Tear Strength
Measurement was performed in accordance with ASTM-D1922 using a light weighted tearing tester (Toyo Seiki Kogyo Co., Ltd.). A 13 mm cut was made into a 64xc3x9751 mm specimen, and then the remaining 51 mm portion was torn, followed by reading the indication.
(7) Refractive Index and Planar Orientation Index (fn)
The refractive index was measured in accordance with the method specified in JIS-K7105 using sodium D ray as light source and using an Atago Type 4 Abbe refractometer. Methylene iodide was used as mounting liquid, and the measurement was performed at 23xc2x0 C. and 65% RH.
The planar orientation index (fn) was calculated from the refractive index measurements using the following equation.
Planar orientation index=(nMD+nTD)/2xe2x88x92nZD
(8) Birefringence
A Berek compensator combined with a NIKON polarization microscope was used to determine the film""s retardation (R), and the birefringence (xcex94n) was calculated by the following equation.
(9) Density and Crystallinity
The density of film was measured with the density gradient method specified in JIS-K7112 using aqueous sodium bromide solution. This density was used in combination with the crystal density and amorphous density of polyester to calculate the crystallinity by the following equation.
Crystallinity (%)=[(film densityxe2x88x92amorphous density)/(crystal densityxe2x88x92amorphous density)]xc3x97100
For PET, amorphous density: 1.335 g/cm3 
crystal density: 1.455 g/cm3 
(10) Heat Shrinkage Starting Temperature
A 4 mm wide specimen cut out from the film was set in the TMA equipment used in paragraph (4) so that the effective specimen length was 15 mm. With a load of 1 g applied, the specimen was heated up to 120xc2x0 C. at a heating rate of 2xc2x0 C./min., followed by determination of the shrinkage (%). The data were recorded to provide graphs of temperature and shrinkage. The temperature at the point where the shrinkage curve departed from the baseline (0%) was read, and this reading was used as the heat shrinkage starting temperature.
(11) Heat Shrinkage
Measurement was carried out in accordance with JIS-C2318. Sample size: width 10 mm, marked line interval 200 mm Measuring condition: temperature 80xc2x0 C., processing time 30 min, unloaded condition
80xc2x0 C. heat shrinkage was calculated by the following equation.
Heat shrinkage (%)=[(L0xe2x88x92L)/L0]xc3x97100
(12) Glass Transition Temperature Tg and Melting Point Tm
A differential scanning calorimeter, Seiko Instruments Inc. Robot DSC-RDC220, was used with a data analyzer, Seiko Instruments Inc. Disk Session SSC/5200. A 5 mg specimen was taken, heated from room temperature to 280xc2x0 C. at a heating rate of 20xc2x0 C./min., maintained at the temperature for 5 min., quenched with liquid nitrogen, and heated again from room temperature to 280xc2x0 C. at a heating rate of 20xc2x0 C./min., and the heat curve obtained was used to determine Tg and Tm.
(13) Center Line Average Surface Roughness (Ra)
Measurement was carried out with a Kosaka Laboratory Ltd. ET-10 high-precision thin-film step measuring machine, and the center line average surface roughness (Ra) was determined according to JIS-B0601. The measurement was performed under the conditions of a tracer tip diameter of 0.5 xcexcm, tracer pressure of 5 mg, measuring length of 1 mm, and a cut-off of 0.08 mm.
(14) Frequency of Film Breakage
For the process for producing biaxially oriented polyester film, the frequency of film breakage was determined according to the following criteria.
Film specimens falling under ⊙, ◯, or xcex94 according to the above criteria were judged xe2x80x9cacceptedxe2x80x9d from the viewpoint of film forming stability and yield.
(15) Surface Damage During High-speed Travelling
Film was slit into xc2xd inch wide strip specimens, which were allowed to run on a guide pin (surface roughness Ra=100 nm) in a tape running test machine (travelling speed 250 m/min., number of times of travelling 1, winding angle 60xc2x0, travelling tension 90 g).
After the travelling of the film had finished, the guide pin was observed visually, and the film was judged excellent (◯) if no debris was found on the pin, good (xcex94) if a small amount of debris was found on the pin, and no good (X) if a large amount of debris was found on the pin. Being excellent (◯) is desired, but films judged good (xcex94) can serve for practical uses.
(16) Electromagnetic Conversion Property (C/N)
The surface of specimens of films of the present invention was coated with a magnetic paint and a non-magnetic paint (coating material) with the following compositions using an extrusion coater (upper coat: magnetic, 0.1 xcexcm thick; lower coat: non-magnetic, varied thickness), magnetically ordered, and dried. Then the opposite surface was coated to form a back coat with the following composition, followed by calendering in a small test calendering machine (steel/steel roll, 5 steps) at a temperature of 85xc2x0 C. and a linear load of 200 kg/cm, and curing at 60xc2x0 C. for 48 hours. Another 8 mm wide strip specimen was cut off from the original film to form a pancake, from which a 200 m tape was taken and set in a cassette to produce a videotape cassette.
A commercial Hi8 videotape recorder (SONY EV-BS3000) and the C/N (carrier/noise ratio) for 7 MHz+1 MHz was measured.
(17) Properties of Heat Transfer Ribbon for Color Printing
The film obtained was coated with ink layers of cyan, magenta, and yellow, to produce a printer ribbon, which was used with a variable dot type heat transfer color printer to print a standard color pattern, followed by visual evaluation of its gradient. Wrinkles in the ribbon were also checked based on the visual observation of the uniformity of the printed part.
(18) Evaluation of Capacitor Properties
A. Dielectric Characteristics
Aluminum was vapor-deposited up to a thickness of 600-1000 xc3x85 over a 18 mm-diameter circular part of both sides of the film to produce a specimen, which was left in an environment of a temperature of 20xc2x15xc2x0 C. and a relative humidity of 65xc2x15% for 48 hours or more. A dielectric characteristics measuring machine, TA Instruments DEA-2970, was used to determine the temperature dependence of dielectric loss tangent at a frequency of 1 kHz and heating rate of 2xc2x0 C./min., and specimens were judged xe2x80x9cgoodxe2x80x9d if they had a dielectric loss tangent of 1.3% or less at the temperature of 105xc2x0 C.
B. Dielectric Breakdown Voltage
Film free of metal deposition is used as specimen for evaluation which was performed according to JIS-C2319.
A rubber sheet with a Shore hardness of 60xc2x0 and a thickness of about 2 mm is spread over a metal sheet of an appropriate size, which is provided with a stack of 10 sheets of aluminum foil of a thickness of 6 xcexcm to serve as lower electrode and a 8 mm-diameter brass cylinder of about 50 g having a smooth, flawless bottom with its 1 mm peripheral part rounded to serve as upper electrode. Prior to the test, the specimen was left in an environment of a temperature of 20xc2x15xc2x0 C. and a relative humidity of 65xc2x15% for 48 hours or more. The specimen was placed between the upper electrode and the lower electrode in an environment of a temperature of 20xc2x15xc2x0 C. and a relative humidity of 65xc2x15%, and a DC power source was used to apply a DC voltage between the electrodes, which was increased from 0V at a rate of 100 V/sec until dielectric breakdown took place. A total of 50 specimens were tested. The dielectric breakdown voltage of each specimen was divided by its thickness, and its average was calculated. Specimens with an average of 400 V/xcexcm or more were judged xe2x80x9cacceptedxe2x80x9d.
(19) Travelling Durability and Preservability of Magnetic Tape
One surface of film of the present invention was coated with a magnetic paint with the following composition up to a thickness of 2.0 xcexcm, magnetically ordered, and dried. Then, the other surface was coated with a back coat layer with the following composition, followed by calendering and curing at 60xc2x0 C. for 48 hours. Another xc2xd inch wide strip specimen was cut off from the original film and processed into a magnetic tape, from which a 200 m portion was taken and incorporated in a cassette to produce a videotape cassette.
The tape was allowed to run for 100 hours in an IBM xe2x80x9cMagstar 3590xe2x80x9d MODEL B1A tape drive, and its travelling durability was evaluated according to the following criteria.
After recording data on the tape in the video cassette using the IBM xe2x80x9cMagstar 3590xe2x80x9d MODEL B1A tape drive, the video cassette was left in an environment of a temperature of 40xc2x0 C. and a relative humidity of 80% for 100 hours, and subsequently data were reproduced to evaluate the preservability of the tape.